Carbon supported polyoxometalates as recyclable solid acid catalysts in aqueous reactions

Acidic Polyoxometalates have been identified as promising catalysts for the hydrolysis of lignocellulosic biomass. They possess a high Brønsted acidity but are soluble in water and most organic solvents, which impedes their recycling and thus industrial application. To develop a solid acid catalyst with both good activity and recyclability, phosphotungstic acid (PTA) was immobilized on activated carbon (AC) by equilibrium impregnation. Characterization of the catalyst materials revealed a stabilizing effect by oxygen functional groups on the support material by increasing the polarity of the support and enabling a chemical interaction between the heteropolyacid and the surface. PTA/AC was applied in the hydrolysis of the hemicellulose xylan as an aqueous model reaction, selectively yielding the biorefinery platform compound xylose with up to 74 % yield. The catalyst can be easily separated from the reaction system and possesses good reusability over 20 recycling runs with a low constant leaching of 3–4 % per run.